In subterranean wells there is an increasing demand for monitoring and controlling downhole devices and systems, for example sensors and instrumentation or control devices. There is a trend towards attempting to bring such instrumentation closer and closer to the actual hydrocarbon reservoir regions and towards multi-well exploration techniques for enabling the extraction of a higher fraction of the hydrocarbon resources available in such reservoirs. At the same time it is desirable to distribute instrumentation and measurement devices at a larger number of locations in order to be able to measure and assess the performance of the well production at more measurement points. A main objective of such measurements is to be able to obtain as accurate measures as possible of the reservoir pressure and temperature, and a secondary objective is to be able to locate possible problems at an early stage.
Hence it is vital to be able to provide systems capable of operating under the relevant conditions and which provides an operator with improved data on the conditions and performance of completed wells.
As an example of the state-of the art, U.S. Pat. No. 6,369,718 B1 to Mathieu describes an oil well monitoring and control system communication network where a twisted pair cable is used to connect well-related locations. A bus interface at a bus supervisor and each node is capable of supporting differential and common mode connection between the bus supervisor and each node, and between the nodes themselves. The interface provides high parallel impedance for differential mode connections and low series impedance for common mode connections. The bus interface comprises a cable having an armored, electrically conducting shell, where the armored shell can be used as the common mode electrical return path.
When trying to extend the solution described in the invention described in US '718 to larger number of nodes, larger depths and longer stretches of signal/power cables, the undesirable effect of increased voltage and power loss in the cable may become a limiting factor for system operation. When connecting more nodes at locations distant from the bus supervisor, the increasing total current requirement results in an increasing corresponding voltage drop down the length of the line. Hence, the supply line voltage at each node will vary depending on actual location along the supply line. Unless the node impedances are made very high, the input power requirement and power efficiency of such a system also may become a limiting factor. If, to the extent possible, the power efficiency is improved by increasing the node impedance, the resulting high impedance signal line will have a tendency to be susceptible to noise, generated internally by switching power demands in the nodes themselves, as well as noise from external sources.
Demands for increasing efficiency in extracting the hydrocarbon resources creates a corresponding demand for obtaining detailed information on the conditions on a well, a demand that can be met by providing more sophisticated measurement and control technology for use in such wells.
It is thus a first objective of the present invention to provide a solution for providing electric power to downhole instrumentation via a network connected to a network controller module. The system provides increased power efficiency in the system by minimizing the applied current, and provides improved communication signal immunity by not letting node current variations influence on the cable voltage and current conditions.
A second objective of the present invention is to provide a power supply system for external downhole instrumentation systems which has low susceptibility to variations in the node power consumption.
A third objective of the invention is to provide a network having high reliability in providing power and communication between downhole instrumentation and control modules and a network control module by reducing the need for components having low reliability (especially at high temperatures), such as large capacitors and inductors.
Yet another objective is to provide a network which allows communication with the nodes in the network while minimizing the resulting disturbances in the power supply.